1. Field of the Invention
The present invention relates to a metal line of a semiconductor memory device, and more particularly, to a multi-layered copper line structure with a dummy via contact and a method for forming the same.
2. Description of the Related Art
As the integration of semiconductor devices is increasing, the width and thickness of metal lines are scaled down. The scaled-down width and thickness causes an increase of resistance, resulting in the decrease of a signal propagation speed. Also, a reduced sectional area of the line can cause a large current density, causing a severe electromigration (EM) of the line. As the size of the semiconductor device becomes sub-micron or less, this phenomenon occurs more often, so that the performance and reliability of metal lines formed of aluminum are greatly degraded.
To solve this problem of the aluminum line, many studies on a copper line have been actively made in recent years. The copper line also has many technical problems to be solved. One of them are voids causing open circuits of the line due to migration of a line material.
A line breakage phenomenon due to electromigration occurs in the copper line less often than in the aluminum line. However, the electromigration is still one of the important factors affecting the reliability of the copper line. In addition, materials of the copper line can migrate due to stress. This phenomenon is known as stress-induced migration (SM). When no current flows due to the decreased line width, a stress is applied to the line due to difference of thermal expansion coefficients between metal and substrate material or insulating material. This stress causes void in the line, resulting in the open circuit of the line.
The void and open circuit formed by the migration of copper atoms due to the electromigration or stress-induced migration generally occur around via contacts that connect a lower copper line to an upper copper line.
Specifically, when the length of the metal line is long, that is, when a distance between the via contacts is long, these defects occur more often.
FIG. 1 is a plan view of a related art multi-layered copper line structure, and FIG. 2A is a sectional view taken along line II-II of FIG. 1.
Referring to FIGS. 1 and 2A, a lower copper line 12 and an upper copper line 16 formed on a substrate 10 are electrically connected together by a via contact 15.
The lower copper line 12, the via contact 15, and the upper copper line 16 are formed in a first interlayer insulating layer 11, a second interlayer insulating layer 13, and a third interlayer insulating layer 14, respectively.
In the case of an aluminum line, the via contact is formed of a material (e.g., tungsten) other than aluminum. However, in the case of a copper line, a damascene process is used. Therefore, the via contact is also formed of copper. Therefore, the void and open circuit due to the migration of copper atoms occurs more often around the via contact 15, which has a relatively smaller volume than the lower copper line 12 and the upper copper line 16.
Specifically, as the length of the copper line 16 is lengthened, the distance between the via contacts 15 are also lengthened so that the defect such as void often occurs in the via.
It can be seen from FIG. 2B that via pollution void can occur as the distance between the via contacts 15 is lengthened.